In a digital still camera market that has been expanding in recent years, there is a growing need for a small and thin camera having greater portability. Circuit components such as LSIs for performing a signal processing have been miniaturized with a high functionality by achieving finer wiring patterns. Also, small recording media with a large capacity have become available at low cost. However, an imaging system constituted by a lens and a solid-state imaging device such as a CCD or a CMOS has not been miniaturized sufficiently, and it is desired that a small imaging system be developed also for realizing a camera with greater portability.
As a configuration for achieving the miniaturization of the imaging system, a configuration using a lens array optical system in which a plurality of microlenses are arranged on a plane has been known. The use of the lens array optical system makes it possible to reduce the thickness of the imaging system in an optical axis direction, and to keep an aberration to be relatively small because individual microlenses have a small diameter.
JP 59(1984)-50042 B discloses an imaging system using such a lens array. This imaging system includes a microlens array having a plurality of microlenses arranged in a plane, a pinhole mask having a plurality of pinholes formed in a plane so as to be in a one-to-one correspondence with the microlenses and an image plane in which light that has passed through each of the pinholes forms an image, in this order from a subject side. Each of the microlenses forms a reduced image of the subject on the pinhole mask, and the individual pinholes allow light corresponding to different portions of this reduced image to pass through (i.e., they sample the light). As a result, a subject image is formed on the image plane.
However, in the above-described imaging system disclosed by JP 59(1984)-50042 B, since the resolution of the subject image formed on the image plane depends on the number and density of the microlenses (namely, the pinholes), it has been difficult to improve an image quality. In other words, because the arrangement of units each constituted by a pair of the microlens and the pinhole determines the arrangement of sampling spots of the image to be obtained, in order to achieve a higher image quality, it is necessary both to increase the number of the above-mentioned units and thus the number of sampling spots and to reduce the size of the individual microlenses and thus an arrangement pitch of the above-mentioned units. However, there is a limit on the miniaturization of the microlenses, so that it has been difficult to achieve a higher resolution. Further, since a light flux reaching the image plane is restricted by the pinholes, there is a significant loss of the light amount, leading to a problem in sensitivity.
JP 2001-61109 A discloses an imaging system using another lens array that solves the problem described above. As shown in FIG. 12, this imaging apparatus includes a microlens array 111 having a plurality of microlenses 111a arranged in the same plane, a partition layer 112 formed of a lattice-shaped partition 112a for separating optical signals from the individual microlenses 111a so as not to interfere with each other and a light-receiving element array 113 having a large number of photoelectric conversion elements 113a arranged in the same plane, in this order from a subject side. One microlens 111a, one corresponding space isolated by the partition 112a and a plurality of the photoelectric conversion elements 113a form one image forming unit 115. In each image forming unit 115, the microlens 111a forms a subject image on the plurality of the photoelectric conversion elements 113a corresponding to this microlens 111a. In this manner, a captured image is obtained for each image forming unit 115. The resolution of this captured image depends on the number of the photoelectric conversion elements 113a (the number of pixels) forming one image forming unit 115. Since positions of the individual microlenses 111a relative to the subject are different, the position at which the subject image is formed on the plurality of the photoelectric conversion elements 113a differs from one image forming unit 115 to another. Consequently, the obtained captured image differs from one image forming unit 115 to another. These plural captured images that are different from each other are subjected to signal processing, thereby achieving a single image.
In this imaging apparatus, since the number of pixels forming each image forming unit 115 is small, the captured image obtained by each image forming unit 115 has a low quality. However, by carrying out the signal processing using the captured images that are obtained respectively by the plural image forming units 115 and shifted slightly from each other and re-forming an image, it is possible to achieve a picture whose image quality is as high as an image captured using a large number of photoelectric conversion elements.